This invention relates in general to electrophotography and, in particular, to a process for preparing electrophotographic imaging members containing a perylene-containing charge generating layer.
In electrophotography, the surface of an electrophotographic plate, drum, belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a print substrate, such as paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material.
In addition, the imaging member may be layered. Current layered organic imaging members have at least a substrate layer and two active layers: (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing electron donor molecules.
The substrate layer may be formed from a conductive material. In addition, a conductive layer can be formed on a nonconductive substrate.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Imaging members with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life.
In the charge transport layer, the electron donor molecules may be in a polymer binder. In this case, the electron donor molecules provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated into the mechanically strong polymer.
Imaging members may also include a charge blocking layer and/or an adhesive layer between the charge generating and the conductive layer. In addition, imaging members may contain protective overcoatings. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
Suitable coating methods used for applying the various layers in electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. Solution coating is a preferred approach because it is more economical than vacuum coating and can be used to deposit a seamless layer.
U.S. Pat. No. 4,855,203 to Miyaka teaches applying charge generating layers from coating solutions comprising a resin dispersed pigment. Miyaka discloses suitable organic solvents for preparing a coating solution of the pigments as including alcohols such as methanol, ethanol and isopropanol; ketones such as acetone, methylethyl ketone and cyclohexanone; amides such as N,N-dimethyl formamide and N,N-dimethyl acetamide; sulfoxides such as dimethyl sulfoxide; ethers such as tetrahydrofuran, dioxane and ethylene glycol monomethyl ether; esters such as methyl acetate and ethyl acetate; aliphatic halogen hydrocarbons such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride and trichloroethylene; or aromatic compounds such as benzene, toluene, xylene, ligroin, monochlorobenzene and dichlorobenzene.
U.S. Pat. No. 3,904,407 to Regensburger et al. teaches applying perylene containing charge generating layers by a vacuum coating process. Vacuum coated charge generating layers containing perylenes show a high photosensitivity. However, vacuum coating is expensive.
Solution coating is a more economical and convenient method of applying charge generating layers. However, perylene pigments are difficult to disperse and unstable dispersions are encountered with coating perylene pigment charge generating layers from solution. Unstable dispersions cause pigment flocculating and settling that leads to coating quality problems. In addition, unstable dispersions are difficult to process, especially in a dip coating process. Further, dip coated perylene containing charge generating layers show a substantial depreciation in photosensitivity as compared to vacuum coated layers.
U.S. Pat. No. 5,521,047 to Yuh et al. is directed to a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer from solution. The process comprises forming a dispersion of a perylene pigment and a polyvinylbutyryl binder in an acetate solvent and applying the dispersion to an electrophotographic imaging member layer by solution coating.
Yuh et al. teaches that perylenes form stable dispersions in acetate solvents for the purposes of application by solvent coating such as dip coating. In addition, Yuh et al. teaches that photoreceptors that include charge generating layers containing perylene charge generating materials applied from dispersions in acetate solvents display increased sensitivity.